What you have here is a passive perfect tense.
Aedificatae sunt means: they have been built. Because the subject, urbs, is plural feminine, the passive perfect participle, aedificatae, is plural feminine too.

Here the subject is the boy. The boy is doing the kicking. This is active voice. It is present tense.

The ball is kicked by the boy.

Here, the same ball is kicked. The tense is present. So what is the difference? The subject is not doing the action. It is being acted upon. Its kind of like it becomes the direct object, but still the subject. The passive voice is formed by taking the 4th principle part of a verb, getting the right gender and number, and adding a form of the verb to be. For example, the perfect tense passive would use the present form of sum. This is just like yours.